GEOMORPHOLOGIC EVIDENCE FOR PLEISTOCENE GROUNDWATER SAPPING ALONG THE FLANKS OF THE JACKSVILLE ESKER, WEST LIBERTY, LAWRENCE COUNTY, PA

The Jacksville Esker extends for 10 km, running uphill to the glacial margin, where a large delta remains from when its conduit discharged into a pro-glacial lake. Exposures of the esker confirm that it is composed of highly permeable, stratified sand and gravel. Along the 600 m section preserved by the Western Pennsylvania Conservancy, we have noted multiple gullies incised into its flanks. One gully on the northern face of the esker presents a morphosequence with a small fan present below it, where the ravine descends upon the surrounding plain. In contrast, the southern face of the esker exhibits two gullies that lack fans. This flank of the esker, however, has been cultivated for many years, which could account for the destruction of once-present fans. We postulate that these gullies resulted from groundwater sapping, as they display the theater shaped headwalls expected. We envision a circumstance during ice margin retreat, with the esker partially exhumed, where meltwater from the adjacent ice front possessed sufficient head to drive the flow of groundwater through the esker to the seepage faces. This process would have occurred shortly after the Last Glacial Maximum. As the gullies incised, displaced gravel would have been winnowed by overland flow, sorting a courser lag deposit in the gully channel, accompanied by fining within the fan from its proximal to distal extent. We intend to continue topographic analyses, coupled to the use of ground penetrating radar, in order to better evaluate our conjectures.